You will first need to find the gun for which you would like unlimited 
ammunition. This trick will only work for one type of gun. After you have 
chosen a gun, find the exact gun on an ally or enemy, then kill him. After 
he is dead, his gun will appear on the ground. When walking over the gun it 
will say you have received that weapon's ammunition. If you have full 
ammunition, waste a few shots then pick up another of that same gun. 
However, this time the gun will stay on the ground. You can keep walking 
over the same gun on the ground repeatedly for more ammunition each time.